1. Field of the Invention
The present invention relates to nuclear magnetic resonance measurements and, more particularly, to methods of improving spectral resolution in the presence of inhomogeneous magnetic fields.
2. Discussion of Related Art
Nuclear magnetic resonance (NMR) spectroscopy is one of the most used methods for the characterization of molecular species, functional groups and structures. The techniques of NMR spectroscopy are well documented in the literature. In general, an NMR apparatus may include an array of permanent magnets that produce a static magnetic field, conventionally called B0, and an NMR antenna (usually including radio frequency (RF) coils) capable of generating an oscillating magnetic field, conventionally called B1. The static B0 and oscillating B1 fields should be substantially perpendicular to one another. The B1 antenna should be capable of transmitting and receiving signals at the Lamor frequency, fL, given by the equation:
                              f          L                ⁢                  •          ⁡                      (                          •                              2                ⁢                •                                      )                          ⁢                  B          0                                    (        1        )            where □ is the gyromagnetic ratio of the nuclear species of interest and B0 is the strength of the static magnetic field. Quantitative NMR measurements may require that the nuclear spins be fully polarized by the static magnetic field prior to data acquisition. The longer the exposure to the static field before the measurement begins, the more complete the alignments of the nuclear moments (spins) by the static field. In general, for the spins to be fully polarized, the exposure time may be approximately three to five times the longitudinal relaxation time T1 of the spins.
Various NMR measurements can be used to distinguish one chemical compound from another. NMR chemical shift is one such measurement. The NMR chemical shift depends on the molecular environment of a spin and is a sensitive function of the electronic structure of molecules. Thus, based on measured chemical shift, chemical conformation may be determined. Crude oil, for example, is a complex mixture of hydrocarbons and NMR spectroscopy may be used to identify hydrocarbon components as well as to distinguish the presence of hydrocarbons from the presence of water. For example, the chemical shift of protons in water is about 4 ppm (parts-per-million), about 1 ppm for aliphatic protons, and about 6-7 ppm for aromatic protons.
The resolution of an NMR spectrum is determined primarily by the inhomogeneity of the external magnetic field. In some existing well-logging tools that include NMR apparatus, such as Schlumberger's Combinable Magnetic Resonance tool (CMR™) and the MRScanner™, the magnetic field may vary by such a degree that the spectral bandwidth is limited by the excitation bandwidth. In other tools, such as the MRILab for the Reservoir Description tool from Haliburton Energy Services, the field inhomogeneity is still likely to be several ppm, or even more than tens of ppm, due to the limited precision in magnet design and construction. Even a very good magnet may have a non-uniformity of about 1%. Such resolution may be insufficient to distinguish between the chemical shifts of water and aliphatic and/or aromatic compounds which differ by only a few ppm.